There has been significant interest in developing laser systems which can be used to treat various forms of vascular lesions. The type of vascular disorders that have been investigated include port wine stains, face veins, telangiectasis, and birth marks. A wide variety of medical laser systems have been proposed and introduced to treat these various disorders.
The prior art lasers were designed to generate an output wavelength which is absorbed by constituents in the blood. When the vein is irradiated, the blood is heated, causing thermal damage to the vein. The damaged vein will thrombose and collapse so that blood will no longer pass through the vein.
The most effective laser systems are designed to deliver a relatively high amount of energy in a short period of time. If the energy is delivered over too long a period, significant thermal damage will occur in regions beyond the vein being treated. In order to avoid this problem and generate higher powers in a short period of time, most prior art systems generated a pulsed output.
One common method of generating short, high energy pulses is to use a Q-switch. In a Q-switched laser, the gain medium is excited during an initial period when lasing does not occur. The Q-switch is then opened, allowing the energy stored in the gain medium to be coupled out of the resonator. Q-switched laser pulses, while having high energy, tend to be relatively short, on the order of tens of nanoseconds. One example of a Q-switched medical laser is disclosed in U.S. Pat. No. 5,217,455, issued Jun. 8, 1993 to Tan. This patent discloses a Q-switched, tunable solid state alexandrite laser which generates an output in the 600 to 1100 nanometer range. The duration of the q-switched pulses is 10 to 300 nanoseconds.
In addition to solid state lasers, tunable dye lasers have also been used for treatment of pigmented lesions. For example, U.S. Pat. No. 5,312,395, issued May 17, 1994, to Tan relates to a dye laser having an output of 345 to 600 nanometers. The patent suggests that the duration of the output pulses should be 500 nanoseconds or less.
In order to provide a wider range of treatment options, it has been suggested that medical laser systems include more than one type of laser. For example, PCT Application No. WO 91/13652, published Sep. 19, 1991, discloses a laser system where both an alexandrite laser and a dye laser are combined in one housing.
It has been recognized that it would be desirable to generate high power pulses having a duration longer than is available in prior art medical laser systems. This problem was addressed in U.S. Pat. No. 5,287,380, issued Feb. 15, 1994, to Hsia. This patent relates to a flash lamp pumped dye laser. A flashlamp power circuit is disclosed which ramps up the amplitude of the drive current in S order to increase the pulse length above 500 microseconds. By using the approach in the Hsia patent, an output pulse of 640 microseconds was created.
The inventors herein believe that the effectiveness of the treatment can be further enhanced if the pulse width can be even further lengthened. More specifically, it is believed that when pulses widths on the order of 500 microseconds or less are used, the laser energy tends to boil the blood in the veins being treated. When the blood is boiled, there is rapid expansion, bleeding and immediate purpura (bruises).
In preliminary investigations, the inventors herein have shown that improved results can be achieved with pulse widths in excess of 500 microseconds. When longer pulse widths are used, the veins tend to be coagulated without boiling the blood. As noted above, there is an upper limit on the ideal pulse width, since longer pulses result in excess thermal damage beyond the treatment site. Therefore, it is believed than an ideal system would be able to generate output pulses having a duration between 0.5 and 10 milliseconds, at a wavelength which is absorbed in the blood and having sufficient power to coagulate the vein.
Accordingly, it is an object of the subject invention to develop a laser system which can generate a long pulse output with sufficient power to coagulate and collapse veins.